1. Field of the Invention
The present invention relates to an optical amplifier for use in optical communications equipment.
2. Description of the Related Art
In recent years, wavelength division multiplexing (WDM) transmission systems have been introduced. In transmitting stations, increasing the number of signal channels results in an increase in the number of post amplifiers used. An optical amplifier takes up a lot of space in optical transmitting equipment. It is therefore desired that the optical amplifier be decreased in size and produced in integrated form.
In the optical amplifier, a laser diode (LD) is used as a pumping light source and a component that gives off heat generated by the LD is associated with the light source. Of components that construct the optical amplifier, the light source and its associated radiator are very large, preventing the optical amplifier from becoming reduced in size. Thus, separating the pumping light source from the optical amplifier and grouping components necessary for use as a pumping light source, such as a light source and its associated driving circuit, as a pumping light source unit were thought of.
FIGS. 1A, 1B and 1C show arrangements of conventional pumping light source units.
As shown in FIGS. 1A, 1B and 1C, three types of pumping light source units may be considered (note that the example of FIG. 1A uses four pumping LDs). The example of FIG. 1C is used in the WDM system developed by CIENA (see catalog "CIENA Multiwave Line Amplifier Block Diagram").
In FIGS. 1A to 1C, 1 denotes a circuit for driving a pumping light source, 2, 3, 4, 5 and 8; pumping light sources, 6; a polarization beam splitter (PBS), 7; a wavelength division multiplexing (WDM) coupler, 9; an optical splitter for separating pumping light, and 10; an optical coupler/splitter for coupling and separating pumping light.
FIG. 1A, each of the four pumping light sources 2, 3, 4 and 5 is driven (for example, supplied with current) by a respective one of the four pumping light driving circuits 1 to produce light. The light from each of the pumping light sources 2 to 5 is output as linearly polarized light. The beams of light output from the pumping light sources 2 and 3 can be set substantially equal to each other in wavelength but will be differently polarized, The beams of light which are substantially equal to each other in wavelength but differently polarized are polarization-coupled by the polarization beam splitter 6. The pumping light sources 4 and 5 are of the same type as the pumping light sources 2 and 3 but will differ from the pumping light sources 2 and 3 in output wavelength. This indicates that, even if the pumping light sources 4 and 5 are of the same type as the pumping light sources 2 and 3, their wavelengths do not necessarily match because of variations in manufacturing process. The polarization-coupled pumping light from the pumping light sources 2 and 3 and the polarization-coupled pumping light from the pumping light sources 4 and 5 are coupled by the WDM coupler 7. The light output from the WDM coupler is sent to an optical amplification medium, for example, an erbium doped fiber (EDF), for use as pumping light for amplifying light signals. The WDM coupler is so named because it couples wavelength-multiplexed signal light and pumping light of a specific wavelength, but in practice it can be an ordinary optical coupler.
This arrangement is used when only one pumping light source cannot provide a sufficient amplification action to the optical amplification medium and intended to obtain pumping light with a larger power through the use of two or more pumping light sources.
The pumping light source unit of FIG. 1B comprises one pumping light source driver 1, one pumping light source 8, and an optical splitter 9 for separating pumping light from the pumping light source 8. This arrangement is used when the pumping light source has a power large enough to supply two or more optical amplification media (not shown). This arrangement allows the two or more optical amplification media to be operated equally with one pumping light source having a single wavelength and a single polarized wave.
The pumping light source unit of FIG. 1C comprises two or more pumping light source drivers 1, an equal number of pumping light sources 8, and an optical coupler/splitter 10 that couples and separates pumping beams of light from the pumping light sources. This arrangement is intended to supply two or more optical amplification media with pumping light from a single pumping light source unit, but it has two or more pumping light sources 8 to provide pumping light with higher power because a single pumping light source alone cannot give all of the optical amplifiers a sufficient amplifying action. As described above, however, there are variations in wavelength between two or more pumping light sources 8. Thus, the use of each of the pumping light sources 8 for a respective one of the optical amplification media will cause their respective amplification actions to vary. For this reason, this pumping light source unit is arranged such that beams of pumping light from the pumping light sources 8 are first coupled to produce a single beam of light and then the single beam of light is separated to thereby supply each of the optical amplification media with pumping light of the same property.
Hereinafter, what includes a pumping light source unit, an optical amplification medium or media, and other circuits including a control circuit shall be called an optical amplifier.
In an optical amplifier, automatic gain control (AGC) or automatic level control (ALC) is sometimes performed to control the supply amount of pumping light to an optical amplification medium. In conventional optical amplifiers, the amount of output light of a pumping light source is varied to vary the supply amount of pumping light to the optical amplification medium by changing a drive current to the pumping light source.
In the WDM transmission system, signal light beams of different wavelengths are collectively amplified by an optical amplifier. After the start of system implementation, the WDM transmission system is sometimes modified to increase the signal multiplexing degree (i.e., the system is upgraded). When the multiplexing degree is increased, the optical amplifier requires more pumping power in order to increase the supply amount of pumping light to the optical amplification media.
In the pumping light source units shown in FIGS. 1B and 1C, the ratio of the output power of each pumping light is fixed to the ratio of the splitting by the splitter 9 or the optical coupler/splitter 10, and the amount of pumping light at each output port cannot be varied arbitrarily.
The optical amplifier contains components that are associated with a pumping light source to radiate heat generated by it. These components are relatively large among components composing the optical amplifier, preventing the optical amplifier from becoming reduced in size. When LDs as pumping light sources that generate heat and driver circuits therefor are placed close together within the optical amplifier, it will create an excessive rise in temperature, reducing the performance and reliability of the optical amplifier.
Heretofore, even if the WDM transmission system is upgraded, pumping light can only be output up to the allowable maximum output of a pumping light source installed in an optical amplifier at the beginning of system implementation. As an example, assume that, in a 16-channel WDM transmission system, only four channels are employed at the beginning of system implementation. In this case, the optical amplifier used is naturally equipped with pumping light sources necessary to accommodate 16 channels, which increases the initial investment at the time of system installation.
In an optical communications device equipped with an optical amplifier of the built-in pumping light source type, heat generated in the device is difficult to radiate and it is therefore necessary to cool the device with a fan, dissipating extra power.
When pumping light of a narrow spectrum width emitted by an LD as a pumping light source reflects from optical parts composing an optical amplifier or a fiber junction back to the pumping LD, the operation of the LD becomes unstable, which makes the operation of the optical amplifier unstable. To avoid this problem, conventionally the optical amplifier has an optoisolater built in on the output side of the pumping LD. This arrangement requires more optical parts.
In the most used type of an optical amplifier, the amplification characteristic of an amplification medium is wavelength-dependent, and pumping LDs have variations in wavelength due to variations in the manufacturing process. For this reason, the optical amplifier has variations in amplification characteristic due to variations in wavelength.
In an optical amplifier in which a pumping light source unit and an optical amplification medium are coupled together by means of a connector, it is necessary to sound an alarm in the case that the connector has come off, because pumping light leaking out through the connector is very dangerous for persons at work.